Total vehicle height is an important design parameter for fitting on existing ship hangers and elevators, as well as fitting inside transport aircraft. As shown in FIGS. 1 and 2A-2B, conventional coaxial helicopters are typically taller than other helicopters of similar performance due to the existence of an additional rotor and the flapping motion of the rotor blades necessitating a minimum vertical hub spacing (rotor separation ratio). In particular, FIG. 1 shows an upper rotor 102 coupled to an upper rotor swashplate 104 and a lower rotor 106 coupled to a lower rotor swashplate 108. A rotor hub separation 110 may generally separate the upper rotor 102 and lower rotor 106. FIG. 2A illustrates a single axis/rotor helicopter 202, whereas FIG. 2B illustrates a coaxial/dual rotor helicopter 204. As shown in FIGS. 2A-2B, the height of the helicopter 204 may be appreciably greater than the height of the helicopter 202.
Some helicopters, such as the Sikorsky X2 Technology™ Demonstrator, may have a reduced rotor separation ratio relative to other helicopters. The reduced rotor separation ratio may be facilitated by the use of hingeless, rigid rotors which may bend rather than flap like articulated rotors do. The high blade rigidity may imply large blade moments and approximately 20% 2/rev blade bending that may increase vibratory (peak to peak) blade stresses beyond 1/rev loads alone. Though 2/rev blade bending may cancel at a hub, the 2/rev blade bending may: (1) decrease minimum blade tip clearance between the two rotors (e.g., rotors 102 and 106), (2) increase peak blade stresses, and (3) increase rotor blade and hub design weight. 2/rev blade control typically cannot be accomplished using an ordinary swashplate, at least for rotors with two blades or more than three blades. In some instances, it may be desirable to utilize a configuration that does not include a swashplate.